3.1. Filtration Performance with Varied Disinfection Treatments
Varied disinfection methods accessible to the general public were investigated for their effects on filtration performance. Without disinfection treatments, both respirators showed very high filtration efficiency of ≥99.5%. It should be noted that the filtration efficiency was measured after challenging the respective aerosols for 3 min (by the KF standard) [35], which corresponded to about 2.7 mg of NaCl and 8 mg of paraffin oil mass. The filtration performance against NaCl and paraffin oil aerosols were comparable, and the effects of various treatments on filtration of either aerosol were very similar (Figure 2).
The resistances of respirators were mostly consistent regardless of treatments, except that laundering treatments slightly decreased the resistance of Resp. A. The physical characteristics of respirators before and after treatment were further investigated in later sections. As for the filtration efficiency, the organic solvents such as IPA and EtOH, and detergent-laundering deteriorated the filtration efficiency of both respirators. Particularly for IPA and EtOH treatments, up to ~28% of efficiency was lost after treatments. Considering that the resistance of the solvent-treated samples was unchanged, it can be inferred that the solvents caused the reduction of filtration performance by affecting the electrostatic filtration capacity, rather than disrupting the structural integrity. Similarly, the filtration efficiency was notably reduced after laundering with detergent. As laundering with water barely affected the filtration efficiency, this indicates that the detergent negatively affected it, probably through an impact on the electrostatic capture mechanism.
Physical treatments employing UV irradiation, oven-dry, and microwave irradiation caused little impact on the filtration efficiency or resistance. Previous studies reported the deteriorated performance with thermal treatment of electret filters [10,41], but most of the performance deterioration occurred with harsher conditions (120 °C, 48 h) and especially for materials with high dielectric constants. Aging the electret filter at an extremely high temperature can trigger the mobility of polymer chains and charge carriers, leading to the loss of charges and increased particulate penetration [41]. In this study, the polypropylene electret filters were treated by the oven-dry condition that was relatively mild and short-term (90 °C, 1 h); and for this reason, the oven-dry treatment caused negligible effect on the performance change. UV irradiation may influence the performance by causing the surface oxidation, turning the surface hydrophilic [42]; and the increased conductivity in a humid condition can lead to the loss of charges, reducing the filtration efficiency [43,44]. However, UV irradiation, at the level of 10 W power with 253.7 nm wavelength, of this study did not significantly affect the filtration efficiency.
The results in Figure 2 show that thermal (oven-dry and microwave) and UV treatments hardly affected the filtration performance, aside from the fact that microwaving is not recommended for the safety reasons with metal components. Samples laundered with water maintained the performance as the untreated ones. However, the probable structural change, the torn coverweb (Figure S3), during the laundering procedure is of concern. Thickness and porosity of filter webs after treatments showed little differences compared to the untreated webs (Table S1). The residues on fibers after detergent-laundering is also of concern in the regard of environmental impacts (Figure S5).